fix fs_open() , now return error code correctly

originally ,it return -2 on error ,now return OPENFAIL (-1)
why it did that is unknown
also ,add indent here and there
delete function fio_getfd & put OPENFAIL in fio.c

fio_getfd() provide bound checking to opened file handle
,but is unused because fio_is_open_int() has already provided that.
also ,OPENFAIL is used to replace -1 literals.